1. Field of the Invention
This invention relates to a piezoelectric flow control valve and a fuel injector incorporating the same.
2. Description of the Related Art
Japanese unexamined patent publication No. 59-206668 published Nov. 22, 1986 and U.S. Pat. No. 4,535,743 issued to Igashira et al. on Aug. 20, 1986, both assigned to the assignee of the present invention, disclose piezoelectric flow control valves as applied to fuel injectors. The flow control valve comprises a body having a fluid inlet and a fluid outlet connected by an internal fluid passage. A movable valve member in the form of a needle valve is accommodated within the body to open and close the internal passage and thus control the outflow of fluid under pressure through the outlet. A plunger integral with the needle valve is slidably received in an axial bore in the body to define a pressure chamber. The hydraulic pressure prevailing at the fluid inlet is transmitted, through a small annular clearance between the axial bore in the body and the outer periphery of the plunger or through a fluid passage in the plunger, to the pressure chamber thereby causing the plunger to bias the needle valve in the closing direction. The pressure chamber is also communicated with a pumping chamber of a piezoelectric pump having a piezoelectric actuator controlled by a control unit. When the piezoelectric actuator is signalled in such a manner as to cause contraction of the piezoelectric stack thereof, the volume of the pumping chamber is increased. This causes a reduction in the hydraulic pressure in the pressure chamber, whereby the needle valve is lifted to allow release of the fluid under pressure through the fluid outlet. Conversely, when the piezoelectric actuator is controlled to cause the piezoelectric stack to expand, a high pressure is developed in the pressure chamber causing the needle valve to move into the closed position. In this manner, the flow control valve is opened and closed by varying the hydraulic pressure acting on the plunger in response to control signals to the piezoelectric actuator.
The piezoelectric flow control valve of this nature is particularly suitable to the control of a fluid flow with a high accuracy, because the piezoelectric actuator operates with a very high responsiveness, compared with a solenoid operated actuator.
However, the problem encountered in the above described flow control valve is that, since the needle valve is arranged to be closed only upon application of the fluid pressure in the pressure chamber onto the plunger, the fluid tends to leak or seep through the fluid outlet of the control valve when the feed pressure at the fluid inlet is zero or is not high enough to tightly close the needle valve.